1. Field of the Invention
The present invention relates to a microporous polyimide sponge and a method for producing the same.
2. Description of the Related Art
Polyimides are used in various applications due to their excellent mechanical properties and good resistance to heat and chemicals. Polyimides can find particular application as special-purpose thermal insulation materials where good heat resistances and chemical resistance are needed. Commercially available polyimide sponge films for thermal insulation are mainly used for energy saving in the chemical industry and other process industries.
Methods associated with the production of polymer sponges are based on the addition of blowing agents, the thermal decomposition of less heat resistant polymers after mixing with more heat resistant polymers, and the addition of ceramic components or porogens.
In addition to these methods based on the addition of pore-forming additives, phase inversion methods using solvents are also known. Wet phase inversion methods are commonly used to produce various polymer sponges. In many cases, wet phase inversion methods using soluble polyimides are applied to the production of polymer films.
On the other hand, a dry phase inversion technique reported by Echiog, et al is also applied to the production of sponge structures from general polyimides whose polyimide backbones are insoluble in solvents. The dry phase inversion technique is easily applicable to polymers of pyromellitic dianhydride and 4,4-oxydianiline. The polymers are, for example, high performance Kapton® (DuPont. Co.) structures. According to the dry phase inversion technique, pyromellitic dianhydride is polymerized with 4,4-oxydianiline in tetrahydrofuran/methanol to prepare a polyamic acid, the polyamic acid is cast into a film, and water is added to induce phase inversion.